crucible question

[R. Scott Johnson]

has anyone here used a AZS crucible? is it better or worse than the regular style? i know they can be really pricey.

thanks for your thoughts

[Steve Stadelman]

Bill Worchester on Maui used to use these A.Z.S. pots that were actually fusion cast (not sintered ground up scraps). He has gone away from them because of the expense and maintenance issues, his had to be invested and temperature control is always a problem with A.Z.S. (ramp rates and all that).

[Pete VanderLaan]

They are very high thermal shock pots. Not recommended for anyone who changes temperature in their furnace by more than 150F per hour.

[R. Scott Johnson]

is it so bad if they are invested? and what is the best investing material?

[Pete VanderLaan]

They will crack and the cracks will open up. Glass will move in and out of the cracks.

The whole notion of a crucible furnace is that you can replace the pot regularly and have great glass while not having to replace the furnace. Investing the pot is a two fold defeat of this notion. First: The pot is stuck in the furnace because of the investment so you rip the crap out of the furnace when you ultimately are willing to spend money on glass quality. Secondly: The crucible is a tool which has a finite lifespan designed in. Investing it is simply a way of trying to extend the lifespan of something that shouldn't really be extended. I have no doubt that people will continue to invest crucibles and will continue to rationalize it somehow. For me, the best material to invest a crucible is simply part of an oxymoron. Now I am quite sure someone will tell you the best material in their opinion.

[Peter Bowles]

Expanded polystyrene?

[Pete VanderLaan]

as good as any.

[Dave Hilty]

When I was first aiming to build a furnace in the late 80's/early 90's most of the models in my neighborhood were the tried and true tank furnace that by definition couldn't be over -insulated because you had to have adequate temp gradient from the inside to the exterior to be sure the glass chilled to seal the seams.

Then, you had lots of folks suggesting investing a pot as this allowed you to gain the advantage of building up the insulation and giving you a lower fuel bill. You also got better glass and the safety of refractory backing up the pot when it gave way or cracked.

To a total newbie, this type of furnace was very attractive because you didn't have the prospect of changing out expensive pots until the glass got so bad that the whole furnace called for a rebuild. The street knowledge was that an invested furnace was likely to last a minimum of several years. Contemplating a free-standing pot furnace seemed way more complicated, expensive and just plain scary. This was a time when "Glass Notes" didn't exist and what was in print was "how to" stuff on tank furnaces.

I guess my point in all this is that a down and dirty, seemingly less difficult build is the invested pot. That was my perception at the time with little or no real experience. All we heard were scary stories about pots cracking & lots of money being spent to replace them. I had no confidence that I could do a build from scratch and make a free-standing crucible deal fly without lots of grief.

Personal financial issues & a little brain surgery intervened to put off my project until ''97. And frankly I took the easy way out and contracted with Charlie Correll to build my recuperated furnace. Even then he had to spend time on the phone with me, convincing me to not go invested because of my lingering "comfort" with that notion.

So Scott, there are a lot of arguments in support of free-standing, most of all great glass. Is there any reason in particular you are considering an invested crucible?

[R. Scott Johnson]

i'm sorry if the thought of an invested AZS crucible is so offensive to you folks.

i'm interested in not just a invested crucible but rather an invested AZS crucible. i have heard stories of glass blowers in New Zealand, John Crocher for one, and other such glass furnaces in australia and Europe that last ten yearrs plus. i am just considering the balance of the loss of down time in changing and cleaning the freestanding pot furnaces, not to mention the cost of ten years of regular crucibles, verses the continual up time of the invested AZS. can anyone tell me the real difference between the invested AZS crucible and a AZS tank? the invested crucible sounds like a no-brainer, at least on the sruface. also considering electric melting which i understand makes the refractories last 2-3 times longer than with gas.

just doing my homework. and i am considering the fusion cast type of crucibles.

[Pete VanderLaan]

Electric melting is easier on crucibles than gas melts. It will not extend the life by two or three times.

Again, melting with AZS assumes that you are not going to allow significant temperature shift in the furnace. The norm max is about 125F per hour or you have trouble. It is very hard to not exceed this shift in a small furnace.

Croucher indeed does use German AZS pots. John melts in an absolutely balls to the walls kind of way frequently melting above 2500F because his silica is really coarse or so he has told me. The pots do not last ten years, that is simply wrong. I don't know how frequently John changes his pots but he changes them fairly frequently to the best of my knowledge.

It currently takes me about three hours to change a crucible in a free standing furnace once every year or so. This does not constitute significant loss of time in my opinion.

But, if you believe it is the way to go, I am not going to convince you otherwise. I doubt you are going to be liking the product of your furnace.

AZS pots can be true castings of the Alumina Zirconium silicate run under common trade names such as monofrax. To the best of my knowledge, Corhart was the last American manufacturer of such products but they shut down their fusion operations and moved them to France. Importing costs will get your attention as well as the exchange rate.

A second type of fusion material is where ground up AZS is cast as a pot using an inferior material to bond it together. Common trade names are things like "Vision" from North American Refractories. Vision is a pretty disappointing product and I have used it in fluorine melters. It falls apart at the casting joints of the inferior product.

You can buy any of these products at a substantial price. I am not offended by your decision to buy something that I don't think will work well for you. I do spend an inordinate amount of time with crucibles of many types and have a pretty fair knowledge of what will and what won't work. My opinion of AZS is that it is a fine product that is not appropriate in a small furnace. If you were to invest AZS and needed a backup castable, my choice would be as high an alumina content castable as I could afford. I like Missouri Refractories "Morco 95" which will run you well over a dollar a pound. Since the AZS is going to crack,the backup castable is your only firewall against real crap coming into the glass thru the cracks, which will be really really large. A cheaper/ lower alumina castable will throw stones at you night and day. Given that both the castable and the AZS will run some serious coin, I really encourage you to ask around before you engage in this venture. I'm not trying to be a snot.

[Jordan Kube]

No way any thing like that is going to last that long. An AZS tank furnace will make nice glass for about three years. You can trudge along after that for awhile but it goes downhill from there.

[Antiny Genet]

I use AZS pots If the image i have attached is there it of my last pot early in its life 8 to 10 melts a hole opened up from bad poor casting i still ran this furnace for 2 1/2 years befor i turned it off.
It took 2 days 1 broken chisel and my hammer drill dose not hammer any more. The back up i used for my invested pot was Ladle bom from Vesuvius after all that time and around 120 melts there was no sign of glass in the back up layer.In the image there is a segment of the pot siting on a hunk of back that had flame impingement it all looked bad but the the furnace was still makeing good glass when i shut it down.
I have had a AZS split slowly over 3 months but this was befor i had a alarm to let me know of flame falure,I came in one morning and the furnace was cold irony it was the day the workman arived to fit my new alarm.

[Michael Mortara]

We got two years from my pot, we blow a lot of glass here, and do a fair bit of casting.
The pot change took a good part of thirty minutes and the furnace body has a fair amount of sag but at five years old (thats about fifty in human years) still has a lot of life left.
As a production blower it just dosn't make sense to go invested when you look at glass quality, turn around time and cost. If you build the right kind of furnace a free standing pot is every bit as user friendly as any invested pot. AZS was designed for the industrial production and for the money might not be the greatest return on your investment.

[Tom Marma]

the round bottom engineered ceramics crucibles are really nice and withstand a lot of abuse and they are reasonably priced. I've had half a pot of glass left and had my furnace crash 3 times which has little insulation and it hasn't cracked. As far as bringing it up to temperature (when the pot is empty) I go 100 degrees per hour to 1000 degrees and then bring it to 1100 over 3 hours, after that I just ramp it up to 2150. I'm using the 30 lb pot and system 96 so I never go above 2200 F.
One thing to consider when having an invested pot is how much corrosion (cords) you might end up getting over time in the glass. Switching your pot out every year or every other year seems to be a good idea for a free standing pedestal type.

[R. Scott Johnson]

thanks all!

[Charlie Correll]

Hold on a minute!

An invested pot furnace is basically a free standing pot furnace with all that lovely combustion space around the pot filled in with a heavy heat sink.

Free standing pots get heat fom the combustion gasses all around the pot, making for much quicker melting. Invested pots, like tanks, have only the surface of the glass into which to drive the heat. think of a tank furnace as a cube of glass. The heat from the flame can only go in through the surface, which is only 1/6 of the total surface of the cube. In the free standing pot, heat goes in the surface, and through the walls of the pot, leaving only the small surface of the pot that sits on the stilt unexposed to flame. And you don't have to heat up all that investment!

You have basically one half the combustion volume in an invested furnace that you have in a free stander. At the same firing rate, the invested furnace combustion gasses have less time to transfer heat to the glass, and that extra heat that does not transfer goes right out the stack. And in an invested, you will have a higher firing rate to make up for the loss.

Free standers are more fuel efficient, make better glass, and the pot change is MUCH easier than on an invested. And no jackhammer required!

The only advantage I see for invested and tanks is that you KNOW you will have glass in the furnace tomorrow. Free standers can crack and drain. But hey, they last a year. When I started with Michael Nourot in 1974, we used pots that lasted eight weeks. That was normal. We still did it. It is SO much better now.

[Pete VanderLaan]

Thank you Charlie. I have tried this tact for the last 20 years and have yet to convince anyone to not invest their pot. They just really really really want that glass there in the morning even if it is a mound of crap. You left out the part about how the brick in a tank is extruded under pressure or cast and has a totally different structure than a crucible does. Simply put the pot has a built in life span by design. If you are contemplating investing, just build a tank.

That being said, free standing crucibles are far better for glass quality.

[Rick Sherbert]

Originally posted by Pete VanderLaan
Thank you Charlie. I have tried this tact for the last 20 years and have yet to convince anyone to not invest their pot.

Not true Pete. It's as a direct result of the information here and the reputation of EC pots that I stopped building invested pot furnaces some years back. It (invested) was what I was shown 17 years ago and I worked in isolation for many years not knowing any better. So you have at least one convert and I suspect others.

[Sky Campbell]

Originally posted by Charlie Correll
Hold on a minute!

An invested pot furnace is basically a free standing pot furnace with all that lovely combustion space around the pot filled in with a heavy heat sink.

Free standing pots get heat fom the combustion gasses all around the pot, making for much quicker melting. Invested pots, like tanks, have only the surface of the glass into which to drive the heat. think of a tank furnace as a cube of glass. The heat from the flame can only go in through the surface, which is only 1/6 of the total surface of the cube. In the free standing pot, heat goes in the surface, and through the walls of the pot, leaving only the small surface of the pot that sits on the stilt unexposed to flame. And you don't have to heat up all that investment!

You have basically one half the combustion volume in an invested furnace that you have in a free stander. At the same firing rate, the invested furnace combustion gasses have less time to transfer heat to the glass, and that extra heat that does not transfer goes right out the stack. And in an invested, you will have a higher firing rate to make up for the loss.

Free standers are more fuel efficient, make better glass, and the pot change is MUCH easier than on an invested. And no jackhammer required!

The only advantage I see for invested and tanks is that you KNOW you will have glass in the furnace tomorrow. Free standers can crack and drain. But hey, they last a year. When I started with Michael Nourot in 1974, we used pots that lasted eight weeks. That was normal. We still did it. It is SO much better now.


Everything you said implies to gas melting. How do you feel this differers when you are melting with electric?

Ok I understand the benefits of freestanding as far as glass quality and in no way trying to change anyones mind.

Here's my story:
First electric furnace we built was a sic melter with a 400plus lb tank made from finly (sp?) block backed with kastolite. (yes I know it's a lousy back up for a tank) This liner crapped out in no time actually sending blocks floating to the surface. This furnace was started at Jax university and when they received the money for a new hub it had to go. So a friend who was a glass major at the time found a warehouse and in it went. Next furnace same crown and basic footprint. Freestanding thick walled pot from another supplier. This pot was just a bit smaller capacity then the tank. Kastolite again this time a port was blocked in with 2800 ifb to facilitate the pot swap. Everything being very similar to the tank furnace backed with the same 10" of fiber over the whole furnace. The only major difference was the increased cubic footage of the inside of the furnace. (The space around the pot)
As you probably already guessed where I'm going (I hear the grown from here)
The electric usage went up with the free stander with less glass over all. The furnace was drawing more power with a freestanding pot over the tank. The scr had to deliver more power to idle the freestander then it did with the tank.
I'm thinking there must be some type of trade off in cubic footage to even heating but I don't have any conclusions. How well does overhead electric melting radiant heat around the pot. How much room or space do you need around the top of the pot to ensure even heating around the pot.
Anyway to this day I still can not convince this glass blower in the benefits of freestanding except for quality of glass. He's currently running 350lb invested sic elements. Basically the same furnace we built here but our is free standing.
I understand better quality glass but seriously what about all the added surface area. There is as much wall space around my pot as in the crown that is double the amount of square inches to heat. Does the thermal flywheel make up for all the added heated refractory? Does the radiant heat really travel down around the pot or is the heat radiating from the pot?


On another note. Many people place there heaters around the freestanding pot. I personally feel this is probably the most economical way to go not to mention the quality of the glass. We still have a lot of people building electric with the heaters in the crown hence the argument.

On the upside to your rolling eyes if I can understand a little better you will never have to hear this from me again.

Here's a heart felt thank you. I've learned heaps from this board and truly appreciate and admire everyones experience knowledge.

[Steve Stadelman]

I am getting great results with heaters above the potline. Great glass, low energy consumption, long heater life......etc.

[Sky Campbell]

Originally posted by Steve Stadelman
I am getting great results with heaters above the potline. Great glass, low energy consumption, long heater life......etc.

No doubt about it. I'm certainly not trying to challenge you or the years of experience on this board. I also built with heaters thru the crown. So that point is moot. As far as low energy consumption that is relative and without a comparison it doesn't matter. I made a side note about heaters around the pot that was not my point. I was just trying to stop that issue from being questioned.

[Steve Stadelman]

Well, I guess I am not sure where you are headed Sky.

You can get great results melting in a freestanding pot with electricity, I don't see any argument. I believe that the use of the shortest heater possible will always be best, therefore sort of negating having them around the pot.

I have seen with a optical pyrometer that the heat does in fact travel pretty uniformly down around the pot to the bottom of the furnace.

[Sky Campbell]

Originally posted by Steve Stadelman
Well, I guess I am not sure where you are headed Sky.

You can get great results melting in a freestanding pot with electricity, I don't see any argument. I believe that the use of the shortest heater possible will always be best, therefore sort of negating having them around the pot.

I have seen with a optical pyrometer that the heat does in fact travel pretty uniformly down around the pot to the bottom of the furnace.

I guess what I'm saying in my personnel experience I have found invested to be more energy efficient and less expensive to run. Not arguing the quality of the glass or maintenance issues. Just cost to run.

This is a quote from my above post to clarify:
"I understand better quality glass but seriously what about all the added surface area. There is as much wall space around my pot as in the crown that is double the amount of square inches to heat. Does the thermal flywheel make up for all the added heated refractory? Does the radiant heat really travel down around the pot or is the heat radiating from the pot?"

especially the question if the heat is radiating down from the heaters to the lower chamber or if the heat is radiating from the pot. I have little to no room around my pot so what's heating the lower chamber the pot or the heaters?

[Charlie Correll]

Sky,

I've been thinking about this ever since I saw your post this morning. Here are a few thoughts. It seems that an electric with elements above the pot would be like a gas furnace fired from the side above the pot level. It's hard to get the heat down, because hot gasses rise. Bottom drafts help. My furnaces are bottom/side fired. I AM surprised that the freestander takes more power. I haven't built any electrics, but it would seem that if the elements dropped down around the pot, the heat would transfer more readily, and you could lower the crown, thus making the chamber smaller, less surface area to lose the heat through.

Does the frax insulation run all the way to the bottom of the walls behind the refractory walls? Some people don't insulate tanks much below the glass line, and if this is true on the furnace you are describing, it may be the culprit. The more a furnace is insulated, the more even the heat is everywhere. 10 inches of frax all the way down does the trick for me. How about under the floor?

[Pete VanderLaan]

The freestanding crucible might take more power but I would bet that the temperature in the lower section of the invested furnace is a lot lower. That's the trade off.

I liken melting in an invested pot to melting in a garbage can. Everytime you charge, the melt moves firther and further from the heat source. Then you have to get the upper furnace way too hot to allow penetration of the mass in the "pot". That means overbuilding the uppers to accomodate the penetration.

Couple that up with the drill and blast to replace the pot, the deteriorating glass quality and it makes no real economic sense to me to do this.

[Sky Campbell]

Thanks Charlie, yes the frax runs all the way to the bottom and it goes 2400 castable then kastolite plus on the bottom.


I just want to clarify it might be more economical to invest.

Let's say I'm right and it's not the heaters that heat the lower section but radiant heat from the pot. If this is true then investing does not make as much of a difference I originally thought.

Has anyone tried packing the lower chamber with frax? I would guess this could be a viable option. Pros Cons ?

Hey Pete,

("The freestanding crucible might take more power but I would bet that the temperature in the lower section of the invested furnace is a lot lower. That's the trade off. ")

What is the trade off? I'm not trying to be a smartass but if the temp is lower down there does it really matter?
If you say yes because of batch melting would this matter if it was a cullet melter?

[Pete VanderLaan]

Originally posted by Sky Campbell
Thanks Charlie, yes the frax runs all the way to the bottom and it goes 2400 castable then kastolite plus on the bottom.


What is the trade off? I'm not trying to be a smartass but if the temp is lower down there does it really matter?
If you say yes because of batch melting would this matter if it was a cullet melter?
********
The trade off is less heat but you pay less money if you are in fact correct. I think it's a wash at best. As to cullet vs batch, the proof is in the final product. Did you get the glass you wanted? My basic objection to invested pots is glass quality, not costs. I do believe that in the long run it's a false economy to invest a pot.

[Charlie Correll]

[QUOTE]Originally posted by Sky Campbell
[B]Thanks Charlie, yes the frax runs all the way to the bottom and it goes 2400 castable then kastolite plus on the bottom.


Sky.

You described that furnace as being 3" of Mizzou castable backed up by 10" of frax. At 2300° on a melt, the calculated skin temperatureof the wall is 163°, and heat loss through that wall is 205 BTU/hr/square foot.

If your floor is 3" of hard refractory castable and 4" of Kastolite 30 (just a guess), then the outside bottom of the floor calculates to be 469°, and the floor is losing 1377 BTU/hr/square foot, almost seven times as much as the walls. This may be the culprit.

[Sky Campbell]

Thanks for entertaining my questions. I just might try the frax packed around the pot to see if it compromises the glass quality. (not that I'm known for the my quality of crystal LOL I use a lot of color). I really need a side buy side comparison to know if the economical difference is worth it. I'll keep you posted.

Charlie you are a vast pool of knowledge thanks for your input. I'm sorry to say I didn't describe the layers very well.
Both furnaces from the ground up were one layer of insalboard brown nasty stuff. That was just a base to pour the next layer of 2400 insulating castable about 4" thick then 3" of kastolite 30 plus. No Frax on the bottom. You can put your hand anywhere on the furnace except the door.

There is also a generous amount of sand uder the pot.

[Garner Britt]

Charlie,

How are cold face temperature calculations made?

I understand the BTU/hr/square foot ratings of various refractories, but how do you estimate what cold face temperatures will end up being?

garner

[Charlie Correll]

AP Green used to publish a booklet entitled "Calculating Heat Transfer Through Refractory Walls." Very analog. Graphs, scales, multiple calculation approximations, interface temperatures. But it worked, and guess what, I lost it! It gave insulation values of various materials as compared to the insulation value of a hardbrick. The term is "equivalent inches of hardbrick." There is a paper on my website called "Recuperation and Insulation: An Overview," which I have presented at Gas Conferences on two occasions.

correllglassstudio.com>> HotGear>> Furnaces>> Recuperation and Insulation: An Overview

DON"T USE correllglass.com !!!!

The paper has tables on the comparative insulation values taken right out of that booklet. It really lets you know how good a material is.

NOW I use THIS which is very digital,

http://heat.hwr.com/

Again, it's AP Green materials, you have to register, but you can do multi layer walls, specifying thicknesses, and the program will calculate everything. It's a little weird at first, but it does the trick. You can figure it out.

Charlie